As is known, proximity sensors for parking lots are available today that are suitable for detecting the presence/absence of a vehicle parked within a delimited area, such as a parking space for example, generally of a rectangular shape.
For example, proximity sensors are known that are based on the use of infrared radiation, or the emission of ultrasounds, suitable for being placed on the overhead ceiling of a parking lot and therefore above the vehicles.
So-called magnetometric proximity sensors are also known, which can be buried in the ground and set up to magnetically interact with the metal masses of vehicles, in order to detect the presence of vehicles.
Proximity sensors are also available of the type described in the Italian patent application entitled “Sensore di prossimitá per area di parcheggio” (“Proximity sensor for parking lots”), filed on Jan. 4, 2009 with application number TO2009A000251. A proximity sensor based on the use of radar is described in this patent application.
In particular, as shown in FIG. 1, this patent application describes a parking lot proximity sensor 1, hereinafter referred to as the proximity sensor 1.
The proximity sensor 1 comprises a radar 2 of the so-called Frequency Modulated Continuous Wave type (FMCW).
As is known, a generic FMCW radar continually emits an electromagnetic wave, which is modulated in frequency by a modulation signal, which has in time a sawtooth or triangular waveform. The thus modulated electromagnetic wave defines an incident signal, such that in the presence of an obstacle, a reflected signal is generated by the reflection of the incident signal on the obstacle in question. The generic FMCW radar then receives the reflected signal and, mixing it with the incident signal, or rather multiplying it with the incident signal, produces a mixed signal spectrum. This spectrum has a plurality of peaks, due to the beats that are generated by the incident signal and the reflected signal. The frequencies where these peaks occur, determinable for example by means of a Fast Fourier Transform (FFT) operation, depend on, in addition to the waveform of the modulation signal, the position of the obstacle with respect to the same generic FMCW radar. The generic FMCW radar is therefore able to determine the distance of the obstacle by performing the FFT operation on the mixed signal.
Having said that, the proximity sensor 1 comprises a signal generator 4, which has a first and a second output, on which it respectively provides a pilot signal p(t) and a timing signal c(t).
As shown in FIG. 2, the pilot signal p(t) is periodic with period Tp(t), equal to 0.1 ms for example, i.e. with frequency fp(t) equal to 10 kHz for example. In particular, the pilot signal p(t) may be a voltage signal having a sawtooth waveform, varying between a minimum voltage −Vmin and a maximum voltage Vmax, with Vmax=|−Vmin|.
The timing signal c(t) is a periodic signal with a period equal to period Tp(t); in addition, the timing signal c(t) can have a square waveform.
Still with reference to the radar 2, it comprises, in turn, a transmitting unit 6 and a receiving unit 8.
In detail, the transmitting unit 6 comprises a transmission stage 10, which is connected via a first input to the first output of the signal generator 4 and generates, on a respective output, a transmission signal si(t) that is frequency modulated on the basis of the pilot signal p(t). In particular, the transmission signal si(t) has a constant amplitude and an instantaneous frequency fi(t) that is a function of the pilot signal p(t). For example, the instantaneous frequency fi(t) may be proportional to the amplitude of the pilot signal p(t) according to the relation:fi(t)=f0+k*p(t)  (1)where k is a constant, while f0 may be, for example, equal to 24 GHz. By way of example, the constant k, minimum voltage −Vmin and maximum voltage Vmax may be such that the instantaneous frequency fi(t) is between a minimum frequency fimin and a maximum frequency fimax, for example, respectively equal to 23.875 GHz and 24.125 GHz. The transmitting unit 6 further comprises a transmission antenna 12 (for example, a patch antenna), connected to the transmission unit 10 in a way to radiate an incident electromagnetic wave I having a constant amplitude and an instantaneous frequency equal to the instantaneous frequency fi(t) of the transmission signal si(t).
Operatively, the incident electromagnetic wave I can impinge on an obstacle 14, if present. In particular, assuming that the proximity sensor 1 corresponds to a respective parking space (not shown), namely that it can detect the presence/absence of a vehicle within this respective parking space, the incident electromagnetic wave I impinges on the obstacle 14, in this case, a vehicle parked in the respective parking space, generating a reflected electromagnetic wave R.
The receiving unit 8 comprises a receiving antenna 16 (for example, a patch antenna), a mixer 18 and a low pass filter 19.
In detail, the receiving antenna 16 is able to receive signals coming from the outside, generating an acquired signal su(t).
The mixer 18 has first and a second input, which are respectively connected to the receiving antenna 16 and to the output of the transmission stage 10, such that the mixer 18 receives as input the transmission signal si(t) and the acquired signal su(t). The mixer 18 then provides a mixed signal sr(t) on a respective output that is equal to the product of the transmission signal si(t) and the acquired signal su(t), i.e. sr(t)=si(t)*su(t).
The low pass filter 19 is connected to the mixer 18, so that it receives the mixed signal sr(t) and generates a filtered signal sf(t). Furthermore, the low pass filter 19 may have a band equal to 150 kHz, for example.
The proximity sensor 1 further comprises an analog-to-digital (A/D) converter 20, which has a first and a second input, respectively connected to the output of the low pass filter 19 and the second output of the signal generator 4, and an output. In addition, the proximity sensor 1 comprises a processing unit 22, of the microprocessor type for example, connected to the output of the A/D converter 20.
In use, the A/D converter 20 samples the filtered signal sf(t) at a sampling frequency fs, generating a plurality of signal samples, namely a sampled signal ss(n). The sampling frequency fs may be a function of the frequency fp(t) of the pilot signal p(t), namely the relation fs=S*fp(t) may hold, where S as a non-integer number, so that fs and fp(t) are relative primes. The sampled signal ss(n) is then received by the processing unit 22.
Operatively, the acquired signal su(t) generated by the receiving antenna 16 varies according to whether the parking space corresponding to the proximity sensor 1 is occupied or not by a vehicle. In particular, in the case where the parking space is not occupied by any vehicle, and therefore in the absence of the reflected electromagnetic wave R, the acquired signal su(t) effectively depends on parasitic couplings present between the transmission antenna 12 and the receiving antenna 16. Vice versa, in the case where the parking space is occupied, the acquired signal su(t) coincides in a first approximation with the reflected electromagnetic wave R. Consequently, the filtered signal sf(t) and the sampled signal ss(n) also vary according to the presence/absence of a vehicle within the parking space.
In practice, the detection of the presence of a vehicle in the parking space corresponding to the proximity sensor 1, namely the determination of the fact that the corresponding parking space is alternatively free or occupied, is entrusted to the processing unit 22.
In detail, the proximity sensor 1 transmits a number NUM of pulses during a calibration step. In particular, these NUM pulses are transmitted during a time interval in which the parking space corresponding to the proximity sensor 1 is not occupied by any vehicle.
In greater detail, during the calibration step, the signal generator 4 transmits a pilot signal p(t) formed by NUM sawteeth, therefore having a duration equal to NUM*Tp(t). The transmission antenna 12 consequently transmits the incident electromagnetic wave I, the instantaneous frequency of which, equal to the instantaneous frequency fi(t) of the transmission signal si(t), defines a number NUM of sawteeth; each pulse is therefore defined by a corresponding portion of the incident electromagnetic wave I, with a duration Tp(t) and with a frequency that follows in time a waveform equal to a single sawtooth. Furthermore, during the calibration step, the proximity sensor 1 acquires a sampled signal ss(n) that, in a first approximation, is a function of just the NUM pulses transmitted, since there is no reflected electromagnetic wave R. The processing unit 22 then calculates a threshold, by adding the thus obtained fs*NUM*Tp(t) samples of the sampled signal ss(n).
In a subsequent detection step, the proximity sensor 1 transmits a further NUM pulses by means of the transmission antenna 12. During this detection step, the proximity sensor 1 acquires a sampled signal ss(n) that is not only a function of the further NUM pulses transmitted, but also of the possible presence of a vehicle in the corresponding parking space.
The processing unit 22 consequently detects the possible presence of a vehicle within the parking space corresponding to the proximity sensor 1 on the basis of the calculated threshold and the fs*NUM*Tp(t) samples of the sampled signal ss(n) obtained during the detection step.
For example, the processing unit 22 can calculate the fs*NUM*Tp(t) absolute differences present between the first and the second samples, where first samples are intended as those samples acquired during the calibration step and second samples as those samples acquired during the detection step. In particular, the processing unit 22 subtracts, in absolute value, each second sample from the corresponding first sample.
Subsequently, the processing unit 22 can add the absolute differences, comparing the result of this addition with the previously calculated threshold and detecting the presence of a vehicle in the case where sum exceeds the threshold by more than a certain percentage of the threshold itself.
Alternatively, the processing unit 22 can calculate an average of the first samples and then calculate, for each first sample, a corresponding absolute deviation, equal to the absolute value of the difference between the first sample and the average. The processing unit 22 then calculates, for each first sample, a respective adaptive threshold, equal to the product of the respective absolute deviation and the respective weight. In addition, the processing unit 22 calculates the absolute differences between the first and the second samples, and then compares each absolute difference with the respective adaptive threshold. In the case where a certain number of absolute differences exceed the respective adaptive thresholds, the processing unit 22 detects the presence of a vehicle.
In practice, the proximity sensor 1 makes use of a radar technology of known type, yet does not need to implement computationally taxing spectrum analysis techniques, but rather detect the presence/absence of a vehicle on the basis of samples of a signal in the time domain, in this case the filtered signal sf(t).
Furthermore, after being inserted inside a suitable container, the proximity sensor 1 can be easily placed in proximity to the ground, as in fact described in patent application TO2009A000251. In particular, the container can be partially buried at a point in the ground that is substantially central with respect to the perimeter of the parking space corresponding to the proximity sensor 1.
The proximity sensor 1 therefore has the advantage that it can also be used, among other things, for monitoring open parking lots, devoid of overhead ceilings. However, the proximity sensor 1 enables detecting whether the corresponding parking space is occupied or not by a vehicle, but does not enable discriminating between different vehicles. Therefore, the proximity sensor 1 can be advantageously employed in applications where it is not necessary to distinguish between different vehicles that occupy the parking space.
On the other hand, in the case where it is wished to implement authorization policies for the occupation of parking spaces, utilization of the proximity sensor 1 may not be sufficient. In fact, such policies require checking that the parking spaces are occupied by effectively authorized vehicles. For example, a typical situation in which an authorization policy is implemented is where it wished to allow use of a parking space only for authorized vehicles, such as police vehicles, ambulances or the vehicles of disabled drivers for example. In this situation, it becomes necessary to discriminate between three different occupation states of the parking space: free, occupied by an authorized vehicle and occupied by an unauthorized vehicle; conversely, the proximity sensor 1 is only able to discriminate between two different occupation states: free and occupied.
Patent FR2917214 describes a survey system on the utilization of parking spaces, which comprises a plurality of sensors buried in the ground, a centralized receiver, a remote receiver, a central processing unit connected to the remote receiver and a plurality of onboard devices, installed on corresponding vehicles. Each sensor is associated with a corresponding parking space and comprises a detection device for the occupation state of this corresponding parking space, formed, in particular, by a magnetic transducer; in addition, each sensor comprises a transmission device, through which it can communicate its own ID and information regarding the occupation state of the corresponding parking space to the centralized receiver, which in turn forwards this information to the remote receiver, and hence to the central processing unit. In greater detail, the transmission device of each sensor can be chosen from a Zigbee, GPRS, Wifi, WiMAX, UMTS or EDGE device.
In practice, the system described in patent FR2917214 envisages that each sensor buried in the ground is equipped with a magnetic transducer and a transmission device, with a consequent increase in the complexity of the system.
Patent application EP1672386 describes the use of an FMCW radar both for detecting the distance and for transmitting data. In particular, according to patent application EP1672386, the signal emitted by the FMCW radar is frequency modulated when the FMCW radar is used to detect the distance, while it is modulated in amplitude, and in particular according to so-called on-off keying modulation, when the FMCW radar transmits data. Similarly, patent application US2007/096885 and the article “FMCW radar with broadband communication capability”, by Peli Barrenechea et al., RADAR CONFERENCE, Jan. 10, 2007, EURAD 2007, pp. 130-133, describe systems in which an FMCW radar is used to transmit data; to this end, the FMCW radar is modulated according to so-called amplitude shift keying (ASK) modulation and amplitude modulation respectively.
Although amplitude modulation (whether of the analog or digital type) of an FMCW radar effectively enables an FMCW radar to be used to transmit bits, every time there is a transition between a ‘0’ bit and a ‘1’ bit, or vice versa, it entails waiting a sufficiently long period of time, typically of the order of milliseconds, before being able to effectively transmit. In fact, when being switched on, every FMCW radar is characterized by a transient of non-negligible duration, during which the frequency and amplitude of the electromagnetic wave it emits undergo unpredictable variations/fluctuations with respect to the frequency and amplitude expected when operating regularly. The described systems are thus characterized by not particularly high bit rates and therefore high consumption.